Plasma arc torches are widely used in the cutting of metallic materials. A plasma arc torch generally includes a torch body, an electrode mounted within the body, a nozzle with a central exit orifice, electrical connections, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. The gas can be non-reactive, e.g. nitrogen or argon, or reactive, e.g. oxygen or air.
In process of plasma arc cutting of a metallic workpiece, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode). The pilot arc ionizes gas passing through the nozzle exit orifice. After the ionized gas reduces the electrical resistance between the electrode and the workpiece, the arc then transfers from the nozzle to the workpiece. The torch is operated in this transferred plasma arc mode, characterized by the conductive flow of ionized gas from the electrode to the workpiece, for the cutting of the workpiece.
In a plasma arc torch using a reactive plasma gas, it is common to use a copper electrode with an insert of high thermionic emissivity material. The insert is press fit into the bottom end of the electrode so that an end face of the insert, which defines an emission surface, is exposed. The insert is typically made of hafnium or zirconium and is cylindrically shaped. While the emission surface is typically planar, it is known to put a small dimple in the end face primarily for centering purposes. For example, Hypertherm manufactures and sells an electrode with an insert having a small dimple in the exposed end face for its 260 ampere oxygen plasma cutting systems.
In all plasma arc torches, particularly those using a reactive plasma gas, the electrode shows wear over time in the form of a generally concave pit at the exposed emission surface of the insert. The pit is formed due to the ejection of molten high emissivity material from the insert. The emission surface liquefies when the arc is first generated, and electrons are emitted from a molten pool of high emissivity material during the steady state of the arc. However, the molten material is ejected from the emission surface during the three stages of torch operation: (1) starting the arc, (2) steady state of the arc, and (3) stopping the arc. A significant amount of the material deposits on the inside surface of the nozzle as well as the nozzle orifice.
The problem of high emissivity material deposition during the plasma arc start and stop stages is addressed by U.S. Pat. Nos. 5,070,227 and 5,166,494, commonly assigned to Hypertherm. It has been found that the heretofore unsolved problem of high emissivity material deposition during the steady state of the arc not only reduces electrode life but also causes nozzle wear.
The nozzle for a plasma arc torch is typically made of copper for good electrical and thermal conductivity. The nozzle is designed to conduct a short duration, low current pilot arc. As such, a common cause of nozzle wear is undesired arc attachment to the nozzle, which melts the copper usually at the nozzle orifice.
Double arcing, i.e. an arc which jumps from the electrode to the nozzle and then from the nozzle to the workpiece, results in undesired arc attachment. Double arcing has many known causes and results in increased nozzle wear and/or nozzle failure. It has been recently discovered that the deposition of high emissivity insert material on the nozzle also causes double arcing and shortens the nozzle life.
It is therefore a principal object of this invention to reduce the nozzle wear by minimizing the deposition of high emissivity material on the nozzle during the cutting process.
Another principal object of the invention is to provide an electrode for a plasma arc torch that results in an improved cut quality.
Yet another principal object of the invention is to maintain the electrode life while providing an electrode that reduces nozzle wear.